This application is based on an application No. 10-9426 filed in Japan, the content of which is hereby incorporated by reference.
(1) Field of the Invention
The present invention relates to an image forming apparatus which forms toner patterns and detects them using a sensor to correct image forming positions in accordance with detection results, and particularly relates to an adjustment to a toner pattern detecting mechanism provided in the image forming apparatus.
(2) Description of the Related Art
A so-called xe2x80x9ctandem-typexe2x80x9d image forming apparatus, as one example of a conventional full-color image forming apparatus, has image holding components (photosensitive drums, for example) set in line corresponding to reproduction colors cyan, magenta, yellow, and black. The reproduction colors are respectively referred to as C, M, Y, and K hereinafter and components related to these colors are assigned numerals with a corresponding C, M, Y, or K. Toner images for different colors formed on the image holding components are sequentially transferred onto a transfer material (a recording sheet, for example) which is transported on a transfer belt or onto the transfer belt as a transfer intermediate component. The toner images are superimposed to form a color image.
In general, for the tandem-type image forming apparatus having the stated construction, a so-called xe2x80x9cregistration correctionxe2x80x9d is performed to correct forming positions of images formed on surfaces of the image holding components corresponding to the reproduction colors. By means of the registration correction, deterioration in the image quality caused by color deviations is prevented.
The following is a brief description of a method of the registration correction, with reference to FIGS. 1A and 1B. FIG. 1A is a diagrammatic illustration of a tandem-type image forming apparatus which has a registration correction mechanism. For the registration correction performed in this image forming apparatus, an optical sensor 925 for optically detecting a toner density is set after photosensitive drums 941C to 941K in the transportation direction of a transfer material, as shown in FIG. 1A. The photosensitive drums 941C to 941K are set along a transfer belt 931. Registration marks 960C to 960K, for example, corresponding to the photosensitive drums 941C to 941K are formed on the transfer belt 931 as shown in FIG. 1B.
The registration marks 960C to 960K are formed in the same shape, and are V-shaped in FIG. 1B. Each of the V-shaped registration marks is composed of a standard line making a right angle with a transportation direction when no deviation is detected and a sloped line forming a 45xc2x0 angle with the standard line. When image writing positions on the photosensitive drums 941C to 941K are correct and transfer positions are also correct, this means that no color deviations occur. In this case, the registration marks 960C to 960K are formed on the exactly same line that is parallel to the transportation direction as shown in FIG. 1B, with the standard lines being formed with a distance D between them in the transportation direction.
The optical sensor 925 set after the photosensitive drums 941C to 941K detects the registration marks 960C to 960K. Due to its detection characteristics, the optical sensor 925 detects a toner density of a point located directly under the optical sensor 925. More specifically, the optical sensor 925 sequentially detects toner densities of points on a dash line shown in FIG. 1B. Here, a high density value is detected at each point of intersection of the dash line and the standard line or the sloped line of the corresponding registration mark 960C to 960K.
If the registration marks 960C to 960K are formed on the transfer belt 931 at respective correct positions, a time period taken from the detection of the standard line to the detection of the next standard line is obtained by dividing the distance D by a moving speed of the transfer belt 931. In addition, time periods respectively taken from the detection of the standard line to the detection of the sloped line of the registration marks 960C to 960K are the same.
Meanwhile, if a timing at which a registration mark is formed is different between the photosensitive drums 941C to 941K, the distance D varies according to the different timings. This means that the time period taken from the detection of the standard line to the detection of the next standard line varies. In this case, an address of image data read from an image memory in the sub-scanning direction is corrected for each pixel so that deviations of the registration marks 960C to 960K are corrected.
If the image forming positions on the photosensitive drums 941C to 941K are deviated in the main scanning direction, the time period taken between the detections the standard line and the sloped line varies with the registration marks 960C to 960K. Since the standard line and the sloped line of each of the registration marks 960C to 960K intersect at a 45xc2x0 angle, relative differences in time periods between the detections of the standard line and the sloped line correspond with deviations of the registration marks 960C to 960K in the main scanning direction. In this case, an address of image data read from the image memory in the main scanning direction is corrected for each pixel, with one of the registration marks 960C to 960K being set as a standard mark. As a result, the time periods respectively taken between the detections of the standard lines and the sloped lines of the registration marks 960C to 960K are the same.
The above operations are performed by a pattern position determining unit 916a, a color deviation calculating unit 916b, and an address correcting unit 916c shown in FIG. 1A. The pattern position determining unit 916a determines timings at which the registration marks 960C to 960K are respectively detected. The color deviation calculating unit 916b calculates the color deviations. In doing so, the color deviation calculating unit 916b obtains the color deviations in the main scanning direction from the relative differences in the time periods respectively taken between the detections of the standard lines and the sloped lines of the registration marks 960C to 960K, and obtains the color deviations in the sub-scanning direction from the relative differences in the time periods respectively taken between the detections of the standard line and the corresponding next standard line. The address correcting unit 916c corrects an address value for each pixel in accordance with the color deviations in the main scanning and sub-scanning directions.
Here, the optical sensor 925 should reliably detect the registration marks 960C to 960K so that the registration correction is correctly performed as stated above. In general, a registration mark has a width equal to or shorter than 8 mm in the main scanning direction. To obtain a valid detection value, about 4 mm middle short part of the 8 mm-wide registration mark needs to be detected. Meanwhile, since tolerances are established for an installation position of the optical sensor 925, a timing at which the optical sensor 925 detects the registration marks 960C to 960K varies according to the position of the optical sensor 925. Also, deviations of the image forming positions of the registration marks 960C to 960K and magnification deviations in the main scanning and sub-scanning directions may be initially great.
When the registration correction is performed, the optical sensor 925 may not be able to reliably detect the registration marks 960C to 960K due to the deviated installation position of the optical sensor 925 and the variations in the image forming positions of the registration marks 960C to 960K which have been determined before the registration correction is performed.
To avoid this problem, the registration mark can be formed in a large size. However, it is desirable to have a distance between the registration marks 960C to 960K as short as possible so that fluctuations in the moving speed of the transfer belt 931 does not adversely affect the formations of the registration marks 960C to 960K. Moreover, to improve accuracy of the registration correction, registration marks need to be formed as many as possible in one correction cycle and a deviation needs to be measured a plurality of number of times. For this reason, the registration marks 960C to 960K should be formed in a small size. It is not desirable to avoid the stated problem by forming the registration marks 960C to 960K in a large size.
The object of the present invention is to provide an adjustment mechanism in an image forming apparatus which forms toner patterns and includes a correction mechanism for correcting image forming positions in accordance with results obtained by the optical sensor that reliably detects the toner patterns by means of the adjustment mechanism.
The object of the present invention can be achieved by an image forming apparatus which corrects a condition for an image formation in accordance with a detection result of a toner pattern formed on a transfer medium transported in a first direction, the image forming apparatus made up of: a specific pattern forming unit for forming at least one specific pattern on the transfer medium; a detecting unit, which is located at a downstream side of the specific pattern forming unit in the first direction, for detecting the specific pattern formed on the transfer medium; a deviation obtaining unit for obtaining a deviation in a second direction between a detecting position on the transfer medium and a predetermined point of the toner pattern to be formed on the transfer medium in accordance with a detection result given by the detecting unit, the second direction intersecting the first direction and the detecting position being a position where the detecting unit detects the toner pattern; and an adjusting unit for adjusting, in accordance with the deviation obtained by the deviation obtaining unit, a positional relation in the second direction between the detecting position and the predetermined point to reduce the deviation.
The object of the present invention can be also achieved by an image forming apparatus made up of: a photosensitive component; a registration mark forming unit for forming a registration mark on the photosensitive component; a specific pattern forming unit for forming a specific pattern on the photosensitive component; a transfer medium on which the registration mark and the specific pattern formed on the photosensitive component are transferred; a sensor for detecting the registration mark and the specific pattern transferred onto the transfer medium; and an adjusting unit for adjusting, in accordance with a detection result of the specific pattern given by the sensor, a positional relation between a detecting position on the transfer medium and a predetermined point of the registration mark to be formed on the transfer medium to reduce a deviation between the detecting position and the predetermined point, the detecting position being a position where the sensor detects the registration mark.
The object of the present invention can be also achieved by an adjusting method of a toner pattern detecting system for an image forming apparatus which corrects a condition for an image formation in accordance with a detection result given by a detecting unit that detects a toner pattern and a specific pattern formed by an image holding component on a transfer medium transported in a first direction, the adjusting method including: a deviation obtaining step for obtaining a deviation in a second direction between a detecting position on the transfer medium and a predetermined point of the toner pattern to be formed on the transfer medium in accordance with a detection result of the specific/pattern, the second direction intersecting the first direction and the detecting position being a position where the detecting unit detects the toner pattern; and an adjusting step for adjusting, in accordance with the deviation obtained in the deviation obtaining step, a positional relation in the second direction between the detecting position and the predetermined point to reduce the deviation.
The object of the present invention can be also achieved by an adjusting method of a registration mark detecting system for an image forming apparatus which forms a registration mark as a toner pattern on a transfer medium, the adjusting method including: a specific pattern forming step for forming a specific pattern as a toner image on the transfer medium; a specific pattern detecting step for detecting the specific pattern formed on the transfer medium using a sensor; and an adjusting step for adjusting, in accordance with a detection result obtained in the specific pattern detecting step, a positional relation between a detecting position on the transfer medium and a predetermined point of the registration mark to be formed on the transfer medium so as to reduce a deviation between the detecting position and the predetermined point, the detecting position being a position where the sensor detects the registration mark.
With these constructions, when the registration correction is performed, for example, a deviation in the main scanning direction between a detecting position of the detecting unit and forming positions of toner patterns (or, registration marks) can be minimized. Consequently, the detecting unit can reliably detect each valid width of the registration marks for the density detection, so that the registration correction can be correctly performed.